Optimization of the structure and operating conditions of dual purpose thermal desalination plants

PIETRASANTA, ARIANA M.; MUSSATI, SERGIO F.; MOROSUK, TATIANA; AGUIRRE, PÍO A.; MUSSATI, MIGUEL C.

Warsaw

Conferencia; 15th CONFERENCE on RESEARCH and DEVELOPMENT in POWER ENGINEERING (RDPE); 2021

Institute of Heat Engineering (Warsaw University of Technology) and the Institute of Applied Research (WUT) in collaboration with the Committees on Energy, Thermodynamics and Combustion, Polish Academy of Sciences.

This paper addresses the optimization of the design of dual purpose thermal desalination plants – simultaneous generation of electricity and production of freshwater –. Most of the large seawater desalination plants in the world are dual-purpose facilities. In these plants, the working fluid in the Rankine cycle (water) is heated in a boiler – if a conventional power plant is considered – or in a heat recovery steam generator (HRSG) – if a combined cycle power plant (CCPP) is considered – to a high energy level and it is passed through steam turbines before the steam is extracted for use at a lower temperature to provide the energy required to run the distillation plants. In the dual purpose plants, the main design specifications are the desired electricity and freshwater demands, which have strong influence on the optimal structure and operating conditions. When the desired electricity demand is more dominant than the desired freshwater demand, the design of the power plant is more important than the design of the desalination plant, and vice versa. When both demands are equally dominant, the designs of power and desalination plants have the same importance. In addition, several structures and designs are possible for the power and desalination plants, leading to a combinatory problem. Then, the systematic determination of the optimal structure and size of dual purpose desalination plants and the operating conditions of each plant´s component is a necessary and challenging task. To this end, a mathematical model involving both discrete and continuous decisions is developed. For electricity generation, a CCHP is considered and modeled by embedding several candidate structures; while for freshwater production two candidate thermal desalination systems are embedded. The model is able to select: a) the optimal configuration of the CCPP, b) the optimal gas turbine (GE Frame 6/39.1 MW or Siemens V 64.3/67.5 MW), c) one or two burners in the HRSG, d) a medium-pressure steam turbine, e) steam reheating at low pressure, f) the best desalination system between a multi-effect distillation (MED) desalination system or a multi-stage flash (MSF) desalination system, g) the optimal operating conditions and sizes of all system´s components. The proposed model is implemented and solved in the General Algebraic Modeling System (GAMS) software. Several case studies considering different demands of electricity and freshwater are presented and optimal solutions discussed.